As a former OS/2 user, is this naming deliberate or co-incidence? And what happenened to OS/3?

Their web site seems to make no mention of OS/2 (I didn't search every page). As with many project web sites, they explain very little about what they are; their home page is mostly project news for people already on board.

Unsolved mysteries of the Universe, No 13 :-
How many remakes of Anna Karenina does the World need?